Retinoblastoma is a malignant pediatric tumor in which mutations in the RB gene occur in the vast majority of cases. While retinoblastoma is initiated by RB inactivation, the steps between RB loss and tumorigenesis are not well understood. This proposal aims to investigate the changes that occur as retinoblastomas initiate and progress to malignancy. Mechanisms of cooperation with RB deletion in retinoblastoma may be broadly relevant for many human cancers. I hypothesize that secondary alterations help the retinoblastoma cell of origin evade a pathway to cell cycle exit controlled by the Rb family member, p130. These secondary alterations may alter the activity of p130 through control of cyclin dependent kinases (CDKs) or may act at other points in the pathway (e.g. by regulating E2F transcription factors). Moreover, I hypothesize that evasion of p130-controlled cell cycle exit in RB-deficient cells may be important not only for retinoblastoma, but for other tumor types. To test these hypotheses, we will use a combination of mouse genetics, cell culture studies and investigation of primary human and murine tumor samples. These studies will use the power of mouse models to determine definitively whether candidate Rb-cooperating genes are important for tumorigenesis and to understand how these co-operating genes synergize with Rb loss. Specific Aim 1: Investigate the mechanism by which Arf functions as tumor suppressor gene in retinoblastoma. Specific Aim 2: Assess whether N-myc alters the activity of the pRB family members in promoting tumorigenesis. This work has important implications for understanding many human tumor types that exhibit RB pathway inactivation. If pathways controlled by RB family members are functionally inactivated through secondary alterations, then reactivation of such pathways may provide new opportunities for therapeutic intervention.